Pigmented inks are used in a variety of industrial and consumer applications. The ink industry has long sought to improve upon the color strength, image gloss, and color density of such pigmented inks, while minimizing the amount of pigment used in each batch of ink. To improve upon these properties, the prior art has provided a number of methods of treating the pigment particles to provide surface-modified pigment particles and to provide pigments with a fine particle size.
Color strength may be determined qualitatively, or may be quantitatively defined according to the CIELAB system of the Commission Internationale de l'Eclairage. In this system, values of L*, a*, and b* are assigned to the color of the ink. L* represents the lightness of the ink with small values down to zero indicating "darkness" and large value up to 100 indicating "brightness." a* represents red or green, with more positive values indicating "redness" and more negative values indicating "greenness." b* represents yellow or blue, with more positive values indicating "yellowness" and more negative values indicating "blueness." See generally Kirk-Othmer Encyclopedia of Chemical Technology 6:523-548 (1979). Gloss may be defined as the percentage of light reflected at the same angle as the angle of incidence. The typically used total angle is 60.degree.. See Kirk-Othmer Encyclopedia of Chemical Technology 16:745 (1981) (describing gloss in the context of paint). Color density is a strength of color which is categorized by black and white, blue (or cyan), red (or magenta) or yellow. A densitometer may be used to measure the strength of darkness, blueness, redness and yellowness in a given ink sample. The color strength, gloss, and density of a pigmented ink may be generally referred to as the color properties of the ink.
The prior art has taught to provide pigment particles having a wetted pigment surface in order to enhance the color properties. See, e.g., B. G. Hays, American Ink Maker (November 1990), which summarizes some of the prior art surface wetting techniques. See also Walter Kurtz, American Ink Maker (June 1987). For instance, acrylic materials are used to wet the surface of the pigment particles in dry form using certain solvents. Use of these materials often results in agglomeration of the pigment particles, and does not reduce particle size of pre-agglomerated pigments. Such agglomeration and large pigment size detract from the color strength and gloss of the pigmented inks.
The prior art further has taught the use of certain ionic materials to modify the surfaces of the pigment particles. For instance, U.S. Pat. No. 4,755,563 discloses block polymers containing ionic moieties such as carboxyl terminated with alkyl or alkyl ether of one to twenty carbon atoms. In addition, the use of ionomers is disclosed in WO93/23795. Such ionic species have been found to be too hydrophilic to be effective in wetting the surface of the pigment particles, inasmuch as the pigment particles will tend to absorb fountain solutions used in printing. This will cause a significant change in the rheological properties of the ink, such as viscosity, yield value, shortness factor, tackiness, and ink transfer efficiency. The ionic materials thus are unsatisfactory in preparing pigmented inks.
The pigments used to prepare inks typically are provided in the form of aqueous slurries. Inks used in commercial processes typically are organic in nature, however, and thus the water present in the slurry must be removed during the process of preparing the ink. To formulate a pigmented, organic ink from the slurry of pigment particles, the slurry is filtered and washed to form a presscake comprising from 20% to 40% by weight pigment particles. This presscake then is further processed to prepare an ink.
The prior art has provided a number of alternative methods for processing the wet presscake of pigment particles to prepare an organic ink. The first method, which may be referred to as the dry powder method, entails drying the presscake at an elevated temperature for several hours to drive off the water. This causes the pigment particles to agglomerate to form a dried presscake comprising a solid mass of pigment. After the presscake has been dried, it then must be milled to render particulate pigment suitable for use in an ink.
A second method, which may be referred to as the flushed paste method, avoids drying the pigment presscake. This method entails mixing the wet presscake with an organic varnish to thereby increase the hydrophobicity of the surfaces of the pigment particles. See generally, Albert Mercado, "Flushes for Paste Inks," American Ink Maker (June-August 1990). The varnishes disclosed by Mercado include a vehicle and a supporting resin, such as pentaeryethrinol ester of rosin. Water separates from the organic varnish phase, and is decanted to leave an ink concentrate. The water remaining in the ink concentrate then is eliminated under reduced pressure with heat. This method has a number of advantages over the dry powder method, in that it avoids the time-consuming and expensive steps of drying the presscake and milling the agglomerated pigment particles to the desired size.
Each of the foregoing methods suffers from a number of disadvantages, however. For example, each method requires the step of filtering the slurry to form a presscake. This step is very time consuming, inasmuch as slurries typically contain from 2%-3% pigment particles by weight. In addition, particle size cannot readily be controlled. The dry powder method does not control pigment particle size at all, and requires milling to render particulate pigment. Nor can particle size readily be controlled in the flushed paste method. Although no milling process is required by this method, the size of the pigment particles in the varnish is typically from about 0.2 .mu.m to about 0.5 .mu.m. This is larger than desired, inasmuch as the ideal particle size for pigmented inks is in the range of about 0.04 .mu.m to about 0.2 .mu.m. Pigment slurries also typically contain high salt concentrations, which might remain in the dried ink concentrate, if the washing process is not complete.
Other methods for enhancing the color strength and gloss of pigmented inks have focused on minimizing the pigment particle size, and preventing agglomeration of the pigment particles. It is well known that color properties are enhanced as particle size decreases. See, e.g., Kurtz, "Aspects of Pigment Processing," American Ink Maker (June 1987). Attempts to enhance color properties by controlling uniformity of pigment particle size also have been made. See, e.g., Albert Mercado, "Flushes for Paste Inks", American Ink Maker (June-August 1990).
Such efforts to improve upon the color properties of pigmented inks as have been described have provided pigmented inks that have acceptable, but not superior, color strength, gloss, and density. Accordingly, a need exists in the art for a method for preparing ink concentrates that have superior color strength, gloss, and density as compared to known pigmented inks. A further need exists for a method for preparing ink concentrates from aqueous slurries of pigment particles, or from dry pigment particles, wherein pigment particle size is controlled. It is a general object of the present invention to provide methods for preparing ink concentrates that will render inks having superior color properties and that satisfy the foregoing needs.